High-voltage lateral gan-on-silicon schottky diode with reduced junction leakage current

ABSTRACT

High-voltage, gallium-nitride Schottky diodes are described that are capable of withstanding reverse-bias voltages of up to and in excess of 2000 V with reverse current leakage as low as 0.4 microamp/millimeter. A Schottky diode may comprise a lateral geometry having an anode located between two cathodes, where the anode-to-cathode spacing can be less than about 20 microns. A diode may include at least one field plate connected to the anode that extends above and beyond the anode towards the cathodes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional application No.62/323,564, filed Apr. 15, 2016, titled “High-Voltage LateralGaN-on-Silicon Schottky Diode” and to U.S. provisional application No.62/323,569, filed Apr. 15, 2016, titled “High-Voltage LateralGaN-on-Silicon Schottky Diode with Reduced Junction Leakage.” Each ofthe foregoing applications is hereby incorporated by reference in itsentirety.

BACKGROUND Technical Field

The technology relates to high-voltage Schottky diodes formed fromgallium-nitride materials.

Discussion of the Related Art

Gallium-nitride semiconductor material has received appreciableattention in recent years because of its desirable electronic andelectro-optical properties. Gallium nitride (GaN) has a wide, directbandgap of about 3.4 eV that corresponds to the blue wavelength regionof the visible spectrum. Light-emitting diodes (LEDs) and laser diodes(LDs) based on GaN and its alloys have been developed and arecommercially available. These devices can emit visible light rangingfrom the violet to red regions of the visible spectrum.

Because of its wide bandgap, gallium nitride is more resistant toavalanche breakdown and has a higher intrinsic field strength comparedto more common semiconductor materials, such as silicon and galliumarsenide. In addition, gallium nitride is a wide bandgap semiconductorand is able to maintain its electrical performance at highertemperatures as compared to other semiconductors, such as silicon orgallium arsenide. GaN also has a higher carrier saturation velocitycompared to silicon. Additionally, GaN has a Wurtzite crystal structure,is a hard material, has a high thermal conductivity, and has a muchhigher melting point than other conventional semiconductors such assilicon, germanium, and gallium arsenide. Accordingly, GaN is useful forhigh-speed, high-voltage, and high-power applications. For example,gallium-nitride materials are useful in semiconductor amplifiers forradio-frequency (RF) communications, radar, and microwave applications.

Schottky diodes typically comprise metal-semiconductor junctions thatexhibit lower threshold voltages than semiconductor-semiconductorjunctions. Because of their lower threshold voltages and much smallerdepletion regions, Schottky diodes can switch from conducting tonon-conducting states much more quickly than semiconductor-semiconductorjunction diodes. In some cases, a Schottky diode can switch severalorders of magnitude faster than a semiconductor-semiconductor diode, andmay operate at terahertz frequencies. Silicon-based Schottky diodes canexhibit reverse-bias breakdown voltages of up to about 100 V.

SUMMARY

Structures and methods for forming high-voltage Schottky diodes withgallium-nitride material are described. In some implementations, thediodes may be formed from one or more layers of gallium-nitride materialdeposited on a substrate of a different material (e.g., silicon orsilicon carbide). The Schottky diodes may be arranged in a lateral,anode-cathode configuration, and may be capable of withstanding reversebias voltages of more than 2000 volts and having low reverse biasleakage currents (e.g., between about 0.4 μA/mm of anode width and about40 μA/mm). The high-voltage diodes may be useful for high-frequencypower electronics, microwave applications including radar, and RFcommunications applications among other applications.

Some embodiments relate to a Schottky diode that comprises agallium-nitride conduction layer, a barrier layer formed adjacent to thegallium-nitride conduction layer, a first cathode and a second cathodespaced apart and formed in electrical contact with the conduction layer,an anode formed adjacent to the barrier layer between the first cathodeand the second cathode, and a gallium-oxide layer formed between theanode and the barrier layer. In some aspects, a thickness of thegallium-oxide layer may be between approximately 1 nm and approximately5 nm.

In some aspects, a Schottky diode may further comprise one or moreanode-connected field plates electrically connected to the anode andextending on opposite edges beyond the anode toward the first cathodeand the second cathode. The one or more anode-connected field plates maycomprise a first anode-connected field plate electrically connected tothe anode and extending on opposite edges beyond the anode toward thefirst cathode and the second cathode by a first distance, and a secondanode-connected field plate electrically connected to the firstanode-connected field plate and extending on opposite edges beyond thefirst anode-connected field plate toward the first cathode and thesecond cathode by a second distance. The second distance may be lessthan the first distance.

In some implementations, a Schottky described above is capable ofwithstanding a reverse-bias voltage of up to 1200 volts. In someaspects, a Schottky diode is capable of withstanding a reverse-biasvoltage of up to 2000 volts. The Schottky diode may exhibit areverse-bias current between approximately 0.4 microamp/millimeter andapproximately 40 microamp/millimeter at a reverse bias of 2000 volts.

According to some aspects, the anode, first cathode, second cathode, anda first anode-connected field plate may be formed of a same material. Insome cases, the anode comprises a multilayer composition selected fromthe following group: Ni/Pd/Au/Ti, Ni/Pt/Au/Ti, Ni/Ti/Al/W, Ni/W/Al/W,W/Al/W, Ni/Wn/Al/W, WN/Al/W, and Pt/Au/Ti. In some implementations, ananode-connected field plate of the one or more anode-connected fieldplates comprises a multilayer composition selected from the followinggroup: Ti/Pt/Au, Al/Cu, or TiN/Cu.

According to some implementations, a Schottky diode may further comprisea buffer layer formed on a substrate, wherein a combined thickness ofthe buffer layer and gallium-nitride conduction layer is at least 4.5microns. The substrate on which a Schottky diode is formed may comprisesilicon. In some implementations, the barrier layer comprises aluminumgallium nitride having a thickness between approximately 10 nm andapproximately 50 nm.

According to some aspects, a Schottky diode may further comprise agallium-nitride cap layer formed over the barrier layer. A thickness ofthe gallium-nitride cap layer may be between approximately 1 nm andapproximately 10 nm.

In some implementations, a Schottky diode may further compriseelectrical isolation regions formed adjacent to the first and the secondcathodes, wherein the electrical isolation regions comprise damagedcrystalline semiconductor that includes one or more of the followingimplanted ion species: nitrogen, phosphorous, boron, and argon.

In some aspects, a Schottky diode may further comprise an insulatinglayer having a thickness between approximately 100 nm and approximately300 nm that is formed between a first extension of a firstanode-connected field plate of the one or more anode-connected fieldplates and the barrier layer. The first extension of the firstanode-connected field plate may extend beyond an outer edge of the anodeby at least one micron. The one or more anode-connected field plates mayinclude a second anode-connected field plate having a second extensionthat extends beyond an outer edge of the first anode-connected fieldplate between approximately 1 micron and approximately 3 microns. Insome aspects, an outer edge of the second extension is spacedhorizontally from the first or second cathode by at least 3 microns. Insome implementations, a distance between an edge of the first or thesecond cathode and an edge of the anode is between approximately 5microns and approximately 25 microns. In some cases, a length of theanode is between approximately 2 microns and approximately 20 microns.

According to some implementations, a Schottky diode as described abovemay be included in a power converter.

Some embodiments relate to a method for making a Schottky diode. Themethod may comprise acts of forming a gallium-nitride conduction layeron a substrate, forming a barrier layer adjacent to the gallium-nitrideconduction layer, forming a first cathode and a second cathode spacedapart and in electrical contact with the conduction layer, forming ananode adjacent to the barrier layer between the first cathode and thesecond cathode, and forming a gallium-oxide layer between the anode andthe barrier layer.

In some aspects, the act of forming the gallium-oxide layer may compriseopening a via to expose a region of a gallium-nitride layer at thelocation of the anode prior to forming the anode, and subjecting theexposed region to an oxygen plasma for a period of time. The period oftime may be between approximately 10 seconds and approximately 120seconds. In some implementations, the act of forming the gallium-oxidelayer may comprise maintaining a pressure during the oxygen plasmabetween approximately 0.5 Torr and approximately 3 Torr.

According to some implementations, a method for making a Schottky diodemay further comprise forming a gallium-nitride cap layer between thebarrier layer and the anode, wherein the gallium-oxide layer is formedfrom the gallium-nitride cap layer. A method may further compriseforming one or more anode-connected field plates in electrical contactwith the anode that extend beyond outer edges of the anode.

The foregoing apparatus and method embodiments may be implemented withany suitable combination of aspects, features, and acts described aboveor in further detail below. These and other aspects, embodiments, andfeatures of the present teachings can be more fully understood from thefollowing description in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the figures, described herein,are for illustration purposes only. It is to be understood that in someinstances various aspects of the embodiments may be shown exaggerated orenlarged to facilitate an understanding of the embodiments. The drawingsare not necessarily to scale, emphasis instead being placed uponillustrating the principles of the teachings. In the drawings, likereference characters generally refer to like features, functionallysimilar and/or structurally similar elements throughout the variousfigures. Where the drawings relate to microfabricated circuits, only onedevice and/or circuit may be shown to simplify the drawings. Inpractice, a large number of devices or circuits may be fabricated inparallel across a large area of a substrate or entire substrate.Additionally, a depicted device or circuit may be integrated within alarger circuit.

When referring to the drawings in the following detailed description,spatial references “top,” “bottom,” “upper,” “lower,” “vertical,”“horizontal,” “above,” “below” and the like may be used. Such referencesare used for teaching purposes, and are not intended as absolutereferences for embodied devices. An embodied device may be orientedspatially in any suitable manner that may be different from theorientations shown in the drawings. The drawings are not intended tolimit the scope of the present teachings in any way.

FIG. 1A depicts an elevation view of structure of a lateral,high-voltage Schottky diode comprising gallium-nitride material,according to some embodiments;

FIG. 1B depicts a plan view of a lateral, high-voltage Schottky diodecomprising gallium-nitride material, according to some embodiments;

FIG. 1C depicts a plan view of multiple anode and cathode contacts for ahigh-voltage Schottky diode comprising gallium-nitride material,according to some embodiments;

FIG. 2 depicts an elevation view of structure of a high-voltage Schottkydiode having two anode-connected field plates, according to someembodiments;

FIG. 3 depicts an elevation view of structure of a high-voltage Schottkydiode having one anode-connected field plate, for which electric fieldsimulations were carried out;

FIG. 4 illustrates calculated electric field profiles along a GaNconduction layer for the structure depicted in FIG. 3 (location anddirection indicated approximately by the dashed arrow) at tworeverse-bias potentials, according to some embodiments;

FIG. 5 illustrates calculated electric field profiles along a GaNconduction layer for the structure depicted in FIG. 3 for twofield-plate lengths at a reverse-bias potential of 500 volts, accordingto some embodiments;

FIG. 6A illustrates the value of the first electric field peak E₁ nearthe anode edge in the GaN conduction layer as a function of siliconnitride thickness for four different field-plate lengths;

FIG. 6B illustrates the value of the second electric field peak E₂ belowthe anode-connected field-plate edge in the GaN conduction layer as afunction of silicon nitride thickness for four different field-platelengths;

FIG. 7 depicts an elevation view of structure of a high-voltage Schottkydiode having two anode-connected field plates, for which electric fieldsimulations were carried out;

FIG. 8 illustrates calculated electric field profiles along a GaNconduction layer for the structure depicted in FIG. 7 (location anddirection indicated approximately by the dashed arrow) at tworeverse-bias potentials, according to some embodiments;

FIG. 9A illustrates the value of the first electric field peak E₁ nearthe anode edge in the GaN conduction layer for the structure depicted inFIG. 7 as a function of silicon nitride thickness for four differentfield-plate lengths;

FIG. 9B illustrates the value of the second electric field peak E₂ belowthe first field-plate edge in the GaN conduction layer for the structuredepicted in FIG. 7 as a function of silicon nitride thickness for fourdifferent field-plate lengths;

FIG. 9C illustrates the value of the third electric field peak E₃ belowthe second field-plate edge in the GaN conduction layer for thestructure depicted in FIG. 7 as a function of silicon nitride thicknessfor four different field-plate lengths;

FIG. 10 illustrates the effect of varying the distance between the firstand second field-plate edges on breakdown voltage for twoanode-to-cathode separations;

FIG. 11 depicts an elevation view of structure of a high-voltageSchottky diode having three anode-connected field plates, for whichelectric field simulations were carried out;

FIG. 12A illustrates calculated electric field profiles along a GaNconduction layer for the structure depicted in FIG. 11 (location anddirection indicated by the dashed arrow) at four reverse-bias potentialswhere the third anode-connected field plate extends approximately 2.5microns beyond the second anode-connected field plate;

FIG. 12B illustrates calculated electric field profiles along a GaNconduction layer for the structure depicted in FIG. 11 at fourreverse-bias potentials where the third anode-connected field plateextends approximately 1 micron beyond the second anode-connected fieldplate;

FIG. 13-1A illustrates a multi-layer substrate on which a high-voltageSchottky diode may be formed;

FIG. 13-1B and FIG. 13-1C depict structures associated with acts forforming cathodes of a high-voltage Schottky diode, according to someembodiments;

FIG. 13-1D, FIG. 13-1E, FIG. 13-1F and FIG. 13-1G depict structuresassociated with acts for forming an anode and a first anode-connectedfield plate, according to some embodiments;

FIG. 13-1H depicts structure associated with deposition of a secondinsulating layer;

FIG. 13-1I, FIG. 13-1J and FIG. 13-1K depict structures associated withacts for forming a second anode-connected field plate, according to someembodiments;

FIG. 13-2A illustrates a multi-layer substrate on which a high-voltageSchottky diode may be formed;

FIG. 13-2B, FIG. 13-2C and FIG. 13-2D depict structures associated withacts for forming anode and cathode vias of a high-voltage Schottkydiode, according to some embodiments; and

FIG. 13-2E and FIG. 13-2F depict structures associated with acts forforming an anode, cathodes, and a first anode-connected field plate,according to some embodiments

FIG. 14 depicts a semiconductor device structure and leakage currentpaths associated with a Schottky diode;

FIG. 15 illustrates leakage current characteristics for a test devicestructure;

FIG. 16A depicts passivation of surface states, according to someembodiments;

FIG. 16B depicts formation of isolation regions by ion implantation,according to some embodiments;

FIG. 17 depicts an elevation view of structure of a lateral,high-voltage Schottky diode comprising gallium-nitride material,according to some embodiments;

FIG. 18 depicts a reduction in leakage current due to oxygen plasmatreatment of a gallium-nitride surface prior to forming an anode,according to some embodiments; and

FIG. 19 shows measured reverse-bias characteristics of exemplarySchottky diodes fabricated according to the present embodiments.

Features and advantages of the illustrated embodiments will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings.

DETAILED DESCRIPTION

Microwave and radio frequency (RF) systems often include circuitryarranged to translate a frequency of a signal to a higher or lowerfrequency value. Frequency translation can occur in applicationsinvolving wireless transmission and receiving of signals. For example,data may be mixed onto a high-frequency carrier wave for transmission,and may later be down-converted at a receiver. Some applications mayinvolve generation of a relatively low-frequency voltage or current thatis proportional to an amplitude of a high-frequency (RF or microwave)signal. Often, Schottky diodes are used in such circuits. Some examplecircuits in which Schottky diodes may be used include, but are notlimited to, single-ended mixers, balanced mixers, double-balancedmixers, double-double-balanced mixers, image-reject mixers, subharmonicmixers, image-recovery mixers, phase detectors, bridge-quad mixers,sampling circuits, frequency multipliers, quadrature-phase modulators,and single-sideband modulators and RF receivers.

Schottky diodes may also be used in power-conversion applications. Forexample, Schottky diodes may be used in various types of powerconverters, e.g., in power rectification and/or inversion circuitry.Schottky diodes may also be used for voltage clamping in poweramplification circuits. In some cases, Schottky diodes may be used forsignal sampling, pulse shaping, or fast logic gating. There are a widevariety of uses for Schottky diodes in signal processing, RF, microwave,and power electronics.

A Schottky diode may be characterized by several figures of merit. Onefigure of merit may be an amount of current the diode can handle whenforward biased. Another figure of merit may be an amount of reverse-biascurrent leaked through the diode when the diode is reversed biased.Another figure of merit may be a breakdown voltage of the diode. Abreakdown voltage may be a maximum amount of reverse-bias voltage thatthe diode can withstand before avalanche breakdown and high currentconduction occurs that can destroy the diode.

The inventors have recognized and appreciated that applications relatingto power amplification and RF and microwave systems may benefit fromSchottky diodes having very high breakdown voltages. The inventors haveconceived and developed methods and structures for forming Schottkydiodes with reverse-breakdown voltages that can exceed 2000 volts. Suchdiodes can be used, for example, in high-frequency power electronics andresist high-voltage transients that might otherwise damage the circuits.

An example high-voltage Schottky diode structure is depicted in FIG. 1A,according to some embodiments. A high-voltage Schottky diode 100 may beformed as a lateral diode structure, and include one or more anodes 140having a length L_(a) and one or more cathodes 130 having a length L_(c)are formed on a same side of a substrate 105. A lateral diode structurehas the benefit of not needing through-substrate vias for connecting toa cathode or anode of the device. This can make integration of thehigh-voltage diode into an integrated circuit (IC) an easier task. Ahigh-voltage Schottky diode 100 may comprise a multi-layer structurethat includes a substrate 105, a buffer layer 112, a conduction layer114, a barrier layer 116, and at least one electrically insulatingdielectric layer 120. Some embodiments may, or may not, include asemiconductor cap layer 118, which may be formed of a same material asthe conduction layer 114.

A high-voltage Schottky diode 100 may further include at least oneanode-connected field plate 145, for each anode 140, and may includecathode contacts 160. Edges of the anode 140 and a nearest edge of thecathode or cathodes 130 may be separated by an anode-to-cathode distanceL₁. Outer edges 146 of the anode-connected field plate 145 may extendbeyond outer edges of the underlying anode 140 by a distance L₂.Electrical isolation regions 115 may be formed around one or moreSchottky diodes. In some implementations, an insulating passivationlayer (not shown) may be formed over the anode-connected field plate andcathode contacts.

Some implementations may include additional layers (not shown) withinthe multi-layer structure. For example, there may be one or more layersbetween the substrate 105 and conduction layer 114. These layers mayinclude any combination of the following layers: amorphous dielectric(e.g., silicon nitride, oxide) layer(s) compositionally graded layer(s),and strain-relieving layer(s). Such layers may be included to amelioratestresses arising from deposition of dissimilar materials and/or toimprove electrical performance of the device (e.g., reduce parasiticcapacitance or leakage currents).

In a plan view, a high-voltage Schottky diode 100 may be arranged asdepicted in FIG. 1B or FIG. 1C. The anodes and cathodes may haveextended widths in one direction and run parallel to each other,according to some embodiments. In some embodiments, a high-voltageSchottky diode may include conductive leads 174, 172 (e.g., patternedduring a metallization level) that extend between one or moreanode-connected field plates 145 and one or more anode contact pads 184,and between one or more cathode contacts 160 and one or more cathodecontact pads 182. The contact pads may be significantly larger thandepicted in the drawing, and may be significantly larger than theanode-connected field plates or cathode contacts. In some embodiments,the anode-connected field plates 145, conductive leads 174, and anodecontact pads 184 may be formed from a same metallization level. In someembodiments, the cathode contacts 160, conductive leads 172, and cathodecontact pads 182 may be formed from a same metallization level, whichmay be the same as or different from the metallization level used toform the anode-connected field plates, conductive leads 174, and anodecontact pads.

In some embodiments, the anode 140, cathodes 130, and anode-connectedfield plate material 145 may be formed from different materialcompositions. For example, a cathode may comprise a multi-layerstructure such as, but not limited to, Ti/Al/Ni/Au, Ti/Al/W, orTa/Al/Ta. The anode may comprise, but is not limited to, Ni/Pd/Au/Ti,Ni/Pt/Au/Ti, Ni/Ti/Al/W, Ni/W/Al/W, W/Al/W, Ni/Wn/Al/W, WN/Al/W, orPt/Au/Ti compositions. An anode-connected field plate may comprise, butnot be limited to, Ti/Pt/Au, Al/Cu, or TiN/Cu compositions.

FIG. 2 depicts an alternative embodiment of a high-voltage Schottkydiode. Elements that are common with the embodiment depicted in FIG. 1Aare identified with same reference numbers. The device depicted in FIG.2 comprises a multi-field-plate Schottky diode 200, and includes asecond anode-connected field plate 147 formed above the firstanode-connected field plate 145. The second anode-connected field plate147 is arranged to electrically contact the first anode-connected fieldplate at a central region of the first anode-connected field plate, andto be spaced from the first anode-connected field plate by a secondinsulating layer 122 over a peripheral region 245 of the firstanode-connected field plate 145. The peripheral region may extend froman outer edge of the first anode-connected field plate toward its centerby a distance L₄. The second anode-connected field plate 147 may furtherextend a distance L₃ beyond an outer edge of the first anode-connectedfield plate 145 toward the cathode or cathodes 130.

Dimensions for components of a high-voltage Schottky diode, for eitherof the embodiments depicted in FIG. 1A and FIG. 2, may be as follows.The anode lengths L_(a) may be between approximately 2 microns andapproximately 20 microns. The cathode lengths L_(c) may be betweenapproximately 2 microns and approximately 20 microns. The widths of theanodes W_(a) and cathodes may be between approximately 100 microns andapproximately 1000 microns, though larger and smaller widths may be usedin some cases. The anodes 140 may be spaced approximately midway betweencathodes 130. A shortest distance L₁ between anodes 140 and cathodes 130may be between approximately 5 microns and approximately 50 microns,according to some embodiments. In some cases, L₁ may be between about 5microns and about 25 microns. In yet other embodiments, L₁ may bebetween about 5 microns and about 15 microns.

In further detail, a high-voltage Schottky diode may be formed on anysuitable substrate 105. Example substrates include, but are not limitedto, silicon (Si), silicon carbide (SiC), gallium nitride (GaN), andsapphire. According to some embodiments, the substrate 105 may comprisebulk monocrystalline silicon. In some instances, the substrate maycomprise a semiconductor on insulator (SOI) substrate where thesemiconductor is any of the foregoing mentioned semiconductor substratematerials. The substrate 105 may be in the form of a wafer (e.g., a Sisemiconductor wafer) and have a diameter between approximately 50 mm andapproximately 450 mm In various embodiments, the surface of thesubstrate is monocrystalline, so that a III-nitride (e.g., GaN, AlN,AlGaN, InGaN) or any other suitable crystalline III-V, II-VI, tertiary,or quarternary material may be epitaxially grown from the surface of thesubstrate.

Because there may be a lattice mismatch between the substrate 105 andthe conduction layer 114, one or more transitional layers may be formedon the substrate as buffer layer 112 to ameliorate stress that wouldotherwise develop from the lattice mismatch. The transitional layers maybe formed by epitaxial growth, according to some embodiments. Forexample, any of the transitional layers may be formed using a chemicalvapor deposition (CVD) process or atomic layer deposition (ALD) process.A CVD process may include, but not be limited to, a metal-organicchemical vapor deposition (MOCVD) process. Other deposition processesmay include hydride vapor phase epitaxy (HYPE) or molecular beam epitaxy(MBE). The transitional layers may include at least a first transitionallayer (e.g., AlN) deposited directly on the substrate 105 followed byone or more gallium-nitride material layers deposited on the firsttransitional layer. Examples of transitional layers 112 are describedin, for example, U.S. Pat. No. 7,135,720 and U.S. Pat. No. 9,064,775,which are both incorporated herein by reference in their entirety. Someof the transitional layers may be compositionally graded. A totalthickness of the buffer layer may be between approximately 0.5 micronand approximately 4 microns.

As used herein, the phrase “gallium-nitride material” refers to galliumnitride (GaN) and any of its alloys, such as aluminum gallium nitride(Al_(x)Ga_((1-x))N), indium gallium nitride (In_(y)Ga_((i-y))N),aluminum indium gallium nitride (Al_(x)In_(y)Ga_((1-x-y))N), galliumarsenide phosporide nitride (GaAs_(x)P_(y) N_((1-X-y))), aluminum indiumgallium arsenide phosporide nitride(Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b)N_((1-a-b))), amongst others.Typically, when present, arsenic and/or phosphorous are at lowconcentrations (i.e., less than 5 percent by weight). In certainpreferred embodiments, the gallium-nitride material has a highconcentration of gallium and includes little or no amounts of aluminumand/or indium. In high gallium concentration embodiments, the sum of(x+y) may be less than 0.4 in some implementations, less than 0.2 insome implementations, less than 0.1 in some implementations, or evenless in other implementations. In some cases, it is preferable for atleast one gallium-nitride material layer to have a composition of GaN(i.e., x=y=a=b=0). For example, an active layer in which a majority ofcurrent conduction occurs may have a composition of GaN. Gallium-nitridematerials may be doped n-type or p-type, or may be undoped. Suitablegallium-nitride materials are described in U.S. Pat. No. 6,649,287,which is incorporated herein by reference in its entirety.

According to some embodiments, the conduction layer 114 may comprisegallium nitride (GaN) or any suitable gallium-nitride material. Theconduction layer 114 may be formed by epitaxial growth (e.g., by anMOCVD process), and may be deposited directly on or above the bufferlayer 112. A thickness of the conduction layer may be betweenapproximately 0.5 micron and approximately 4 microns. In someembodiments, the conduction layer may be lightly doped (for either n orp type conductivity) or may be undoped.

The inventors have found that it is desirable to have a combinedthickness of the buffer layer 112 and conduction layer 114 to be atleast 4.5 microns, in some embodiments. This can avoid limiting thedevice's lateral breakdown by the vertical epitaxial profile. In somecases, the combined thickness of the buffer layer and conduction layeris at least 4.0 microns to avoid limiting the device's lateral breakdownby the vertical epitaxial profile.

A barrier layer 116 may be formed using any suitable epitaxial growthprocess, and may be deposited directly on or above the conduction layer114, in some embodiments. A thickness of the barrier layer 116 may bebetween approximately 10 nanometers and approximately 50 nanometers,though other thicknesses may be used in some cases. According to someembodiments, the barrier layer 116 may comprise any suitablegallium-nitride material. The barrier layer may be doped for either n orp type conductivity, or may be undoped. The barrier layer 116 andconduction layer 114 may form a heterojunction, and thereby create atwo-dimensional electron gas (2DEG) in the conduction layer 114 adjacentto the interface between the conduction layer and barrier layer. The2DEG 214 (depicted in FIG. 2) may provide a highly conductive path forcurrent flowing between the anode 140 and cathodes 130.

When using the terms “on,” “adjacent,” or “over” in to describe thelocations of layers or structures, there may or may not be one or morelayers of material between the described layer and an underlying layerthat the layer is described as being on, adjacent to, or over. When alayer is described as being “directly” or “immediately” on, adjacent to,or over another layer, no intervening layer is present. When a layer isdescribed as being “on” or “over” another layer or substrate, it maycover the entire layer or substrate, or a portion of the layer orsubstrate. The terms “on” and “over” are used for ease of explanationrelative to the illustrations, and are not intended as absolutedirectional references. A device may be manufactured and/or implementedin other orientations than shown in the drawing (for example, rotatedabout a horizontal axis by more than 90 degrees.

According to some embodiments, the conduction layer 114 comprisesundoped gallium nitride (GaN), and the barrier layer comprises undopedaluminum-gallium nitride (AlGaN) having an Al percentage (by molefraction) between approximately 20% and approximately 40%.

Some embodiments may include a semiconductor cap layer 118 formed overthe barrier layer 116. The semiconductor cap layer may comprise asemiconductor material of the same type as the conduction layer 114. Thecap layer 118 may or may not be doped. In some implementations, the caplayer may comprise a layer of undoped or doped GaN. The cap layer 118may have a thickness between approximately 1 nm and approximately 10 nm.The cap layer may be formed by any suitable epitaxial deposition process(e.g., by ALD or CVD). Some implementations may not include a cap layer118.

The conduction layer 114, barrier layer 116, and cap layer 118 may havelow defect densities that are typical for integrated-circuit-gradesemiconductor material. For example the defect density for each layermay be less than approximately 10⁹ cm⁻² in some implementations, andless than approximately 10⁸ cm⁻² in some embodiments. Defect densitiesmay be higher in the buffer layer 112 or in portions of the bufferlayer. Any suitable insulating layer 120 may be used to electricallyinsulate one or more anode-connected field plates from the barrier layer116 or cap layer 118. Example insulator materials include, but are notlimited to silicon nitride, silicon oxide, hafnium oxide, aluminumoxide, lanthanum oxide, titanium oxide, zinc oxide, zirconium oxide,gallium oxide, scandium oxide, aluminum nitride, and hafnium nitride. Aninsulating layer may be formed by any suitable deposition process, suchas chemical vapor deposition, plasma-enhanced chemical vapor deposition,atomic layer deposition, or electron-beam evaporation. Other depositionprocesses may be used in other embodiments.

Anodes 140, cathodes 130, anode-connected field plates 145, 147, andcathode contacts 160 may be formed from a metal, a metal silicide, metalalloys, a plurality of metal layers, or a highly-doped amorphoussemiconductor. In some implementations, any of the anodes, cathodes,anode-connected field plates, and cathode contacts may comprise one ormore layers of the following metals and/or metal alloys in any suitablecombination: titanium, nickel, chromium, platinum, palladium, osmium,aluminum, gold, tungsten, rhenium, tantalum, and alloys of titanium andtungsten. In some cases, one or more of the following silicides may beused: platinum silicide, tungsten silicide, nickel silicide, cobaltsilicide, titanium silicide, molybdenum silicide, and tantalum silicide.Any of the anode, cathode, and field-plate elements may be formed by aphysical deposition process (e.g., electron-beam deposition orsputtering or plating). A thickness of a cathode or anode may be betweenapproximately 20 nm and approximately 200 nm, though other thicknessesmay be used in some cases. A thickness of an anode-connected field plate145, 147 may be between approximately 200 nm and approximately 1.5microns. A thickness of a cathode contact 160 may be betweenapproximately 200 nm and approximately 2 microns.

Although only one or few Schottky diode structures are depicted in thedrawings, many Schottky diode structures may be fabricated in parallelon a substrate 105. For example, the substrate 105 may comprise asemiconductor wafer and hundreds, thousands, or millions of thedescribed Schottky diode structures may be fabricated on thesemiconductor wafer. Some diodes may comprise multiple anodes andcathodes connected together, as depicted in FIG. 1C, so that largercurrents can be handled by a diode.

In some implementations, isolation regions 115 may be formed around oneor more Schottky diodes to prevent inflow or outflow of current to orfrom a diode to an adjacent circuit element. Isolation regions maycomprise shallow trench isolation structures (e.g., trenches filled withan oxide or other insulator), in some cases, or may comprise regions ofdamaged crystalline semiconductor in other embodiments. The inventorshave recognized and appreciated that effective isolation regions may beformed in gallium-nitride materials by damaging the crystal latticestructure with ion implantation (e.g., implanting nitrogen, argon,boron, or phosphorus). In some embodiments, an isolation region may beformed around one or more Schottky diodes by implanting a peripheralregion with nitrogen at multiple different energies. The differentimplantation energies are used to extend the damaged region around thediode from the top of the barrier layer 116 (or cap layer if present) toa depth of at least 100 nm. Forming isolation regions 115 by ionimplantation can be easier than process steps associated with forming afield oxide around the diodes.

The inventors have recognized and appreciated that structure relating tothe anode-connected field plates 145, 147, insulating layers 120, 122,anode-cathode spacing L₁, and location of field-plate edges cancritically determine a reverse-bias breakdown voltage for the describedSchottky diodes. To illustrate, several numerical simulations werecarried out to calculate the magnitude of electric fields in differentSchottky diode structures. The structures and results from thesimulations are described in connection with FIG. 3 through FIG. 12B.

FIG. 3 depicts a Schottky diode structure 300 having a singleanode-connected field plate 145 that served as a device model for afirst set of numerical simulations to evaluate electric fields withinthe device under various bias conditions. The structure used in thesimulations comprised a top portion of a Schottky diode, and included aGaN conduction layer 114, an AlGaN barrier layer 116, an anode 140, twocathodes 130, and an anode-connected field plate 145.

A first insulation layer 120 (silicon nitride) was included above theAlGaN barrier layer, and a passivation layer 150 (silicon nitride) wasincluded over the device. For the simulations, a surface-state donordensity at the interface of the AlGaN barrier layer 116 and the GaNconduction layer was 5×10¹²/cm². This surface-state density was found toproduce a 2DEG in the GaN conduction layer and provide diode-likecurrent-voltage device characteristics. The length of the anode L_(a)was 10 microns and the distance L₁ from the anode edge to cathode edgewas 10 microns. In the simulations, the distance L₂ by which the fieldplate edge extended beyond the anode was varied.

In a first set of simulations, the magnitude of the electric field inthe gallium nitride conduction layer was calculated as a function ofdistance from a center of the anode toward the cathode. For thesesimulations, L₂ was approximately 5 microns. In a first case, a bias of−100 V (a reverse bias) was applied between the anode and the cathode.In a second case, a bias of −500 V was applied between the anode and thecathode. Plots of the electric field for each case are shown in FIG. 4.The plots illustrate the electric field values along the dashed arrow ofFIG. 3, and are plotted for one side of the symmetric diode structure.Each plot shows a first peak in the electric field E₁ that appears belowthe edge of the anode, depicted as region 310 in the conduction layer114. A second peak E₂ appears in the electric field below the edge ofthe anode-connected field plate 145, depicted as region 320. Otherreverse bias potentials were also trialed. For these simulations, it wasobserved that the first peak E₁ tends to saturate with increasingreverse bias to a value below 2×10⁶ V/cm. The second peak E₂ thatappears below the outer edge of the field plate 145 increases in valueto over 3×10⁶ V/cm. Since the conduction layer comprises GaN with anintrinsic field strength of 5×10⁶ V/cm, the reverse bias potential maybe increased further (as much as 800 V or higher) before breakdown isobserved.

The anode-connected field plate 145 spreads the electric field in theconduction layer, and helps suppress the first electric field peak E₁that forms at the edge of the anode. Without the anode-connected fieldplate 145, the first electric field peak E₁ would rise to a breakdownvalue well before a reverse bias of 800 V.

A second set of simulations were carried out to investigate differentanode-connected field plate lengths. The results from these simulationsare shown in FIG. 5. For these simulations, a bias of −500 V was appliedbetween the anode and cathode. In these simulations, the extensionlength L₂ of the anode-connected field plate was changed from 1 μm to7.5 μm. In the first simulation, shown as the dotted line, the outeredge of the anode-connected field plate was located about 1 μm beyondthe outer edge of the anode. For this case, the first peak E₁ in theelectric field was just below 3×10⁶ V/cm, and the second peak E₂exceeded 3×10⁶ V/cm. Other extension lengths L₂ trialed included 2.5 μm,5 μm, and 7.5 μm. It was found that increasing the extension length ofthe anode-connected field plate has little effect on the magnitude ofthe second electric field peak E₂. For example, in the fourth trial,shown as the solid line, the field-plate extension length was 7.5 μm,and although the value of the first electric field peak reduced, thesecond electric field peak E₂ remained at approximately the same value,just over 3×10⁶ V/cm.

Additional simulations were carried out to observe changes in themagnitudes of the electric field peaks E₁ and E₂ due to varying thethickness t₁ of the insulating layer 120. Additionally, field-plateextension lengths L₂ were also varied in the simulations. For thesesimulations, the length of the anode was 10 μm, and a distance from theanode to the cathode was also 10 μm. Also a bias of −500 V was appliedbetween the anode and cathode. The insulating layer 120 comprisedsilicon nitride. The observed changes in electric field peaks areplotted in FIG. 6A and FIG. 6B.

In FIG. 6A the value of the first electric field peak E₁ is plotted as afunction of insulating layer thickness t₁ for four different field-plateextension lengths L₂. A first trace 610 was observed for a field-plateextension length of 0.5 μm. A second trace 620 corresponds to afield-plate extension length of 1 μm. The third trace 630 and forthtrace 640 correspond to field-plate extension lengths of 2.5 μm and 7.5μm respectively. As can be observed, the magnitude of the first electricfield peak E₁ decreases with decreasing thickness in the insulatinglayer 120. Additionally, the magnitude of the first electric field peakE₁ decreases with increasing length of the field-plate extension lengthL₂.

FIG. 6B illustrates plots of the second electric field peak E₂ as afunction of insulating layer thickness t₁ for the same field-plateextension lengths that were trialed for the graph of FIG. 6A. In termsof the second peak E₂, the extension length L₂ of the field plate haslittle effect on the value of the second electric field peak. Thisresult is consistent with the results shown in FIG. 5. Additionally, thevalue of the second electric field peak E₂ decreases with increasingthickness t₁ of the insulating layer 120. This is an opposite trend fromthat observed in FIG. 6A. These results suggest that an insulating layerthickness for a single field-plate design is preferably in a rangebetween about 100 nm and about 300 nm. The results also indicate that isbeneficial to have the first anode-connected field plate 145 extendbeyond the outer anode edge by at least one micron.

A second set of simulations were carried out to calculate the electricfield values in the gallium-nitride conduction layer for a twofield-plate design depicted in FIG. 7. The structure 700 included asecond anode-connected field plate 147 electrically connected to a firstanode-connected field plate 145. The second anode-connected field plateextended farther toward the cathode than the first anode-connected fieldplate by a distance L₃. The second anode-connected field plate may beformed above and in electrical contact with the first anode-connectedfield plate. For this design, a second insulating layer 122 overlies thefirst anode-connected field plate and cathodes, and portions of thesecond anode-connected field plate are located above the secondinsulating layer 122. A passivation layer 150 was included over thedevice, and the surface-state density was 5×10¹²/cm². The length of theanode was 10 μm, and the distance from the anode edge to the cathodeedge was also 10 μm. For the simulations, a thickness of the firstinsulating layer 120 was fixed at 50 nm. The extension length L₂ of thefirst field plate 145 was fixed at 2.5 μm.

Electric field values along the gallium-nitride conduction layer werecalculated for different reverse bias potentials, of which two are shownin FIG. 8. The extension length L₃ of the second field plate was fixedat 2.5 μm for the results shown in FIG. 8. In a first trial the appliedbias was −100 V, and the value of the electric field along thegallium-nitride conduction layer 114 is shown as the dashed line 810. Ina second trial, the applied bias was −500 V and is shown as the solidtrace 820. In each case, three peaks in the electric field were observedin the gallium-nitride conduction layer between the anode center and thecathode edge. The first peak E₁ corresponds to an outer edge of theanode, located at approximately 5 μm in the simulated structure 700. Thesecond peak E₂ appears below the outer edge of the first anode-connectedfield plate 145. A third peak E₃ appears below the outer edge of thesecond anode-connected field plate 147, depicted as region 330 in theconduction layer 114. Similar to the single field-plate design, thevalue of the first electric field peak E₁ saturates with increasingreverse bias potential. For this simulated structure, the value of thefirst electric field peak E₁ reaches about 1.4×10⁶ V/cm. The values ofthe second and third electric field peaks, however, increase withincreasing reverse bias voltage. For the simulated structure, the valuesof the second and third electric field peaks reach about 2.2×10⁶ V/cm ata reverse bias of about 500 V. Adding a second anode-connected fieldplate 147 reduces the value of the peak electric fields beyond the edgeof the anode, as can be seen by comparing the plots of FIG. 8 with thoseof FIG. 4.

In additional simulations, the extension length L₃ of the second fieldplate was varied between the following values: 0.5, 1.0, 2.5, and 5.0μm. Results from these simulations were similar to those shown in FIG.5, but with an additional electric field peak E₃.

Simulations were also carried out to observe changes in the peakelectric fields E₁, E₂, and E₃ caused by changing the thickness t₂ ofthe second insulating layer 122. The results from these simulations areshown in FIG. 9A through FIG. 9C. In FIG. 9A the value of the firstelectric field peak E₁ is plotted as a function of total insulatorthickness. The total insulator thickness comprises the fixed thicknessof the first insulating layer 120 (t₁=50 nm) and the thickness t₂ of thesecond insulating layer 122, which was varied for each case. Fourfield-plate extension lengths L₃ for the second anode-connected fieldplate 147 were trialed. As can be seen from the plots of FIG. 9A,changes in thickness t₂ of the second insulating layer 122 and changesin extension length L₃ of the second anode-connected field plate havelittle effect on the magnitude of the first electric field peak E₁.

FIG. 9B, however, shows that changes in insulator thickness t₂ andchanges in extension length L₃ change the value of the second electricfield peak E₂. For the simulated structure 700, increasing thefield-plate extension length decreases the value of the second electricfield peak, as does decreasing the thickness of the second insulatinglayer 122. The first trace 910 corresponds to a field-plate extensionlength L₃ of 0.5 μm. The second trace 920 corresponds to a field-plateextension length of 1 μm. The third trace 930 and fourth trace 940correspond to a field-plate extension length of 2.5 μm and 5 μm,respectively.

The value of the third electric field peak E₃ is plotted in FIG. 9C as afunction of the total insulator thickness (t₁+t₂) for the same forfield-plate extension lengths used in FIG. 9A and FIG. 9B. As can beseen, increasing the field-plate extension length has little effect onthe value of the third electric field peak E₃. However, increasing thethickness t₂ of the second insulating layer decreases the value of thethird electric field peak, which is an opposite trend from that observedin FIG. 9B. Because the value of the second electric field peak E₂remains below a breakdown field strength for gallium nitride at largerthicknesses of the second insulating layer 122, it may be preferable touse a thicker insulating layer for the second insulating layer to keepthe second and third electric field peak values in the gallium-nitrideconduction layer below a breakdown field strength. For example, thesecond insulating layer 122 may have a thickness t₂ betweenapproximately 400 nm and approximately 600 nm, according to someembodiments.

Based upon the simulation results, a number of high-voltage Schottkydiodes were fabricated according to structure shown in FIG. 7. Thediodes included two anode-connected field plates formed above and anode.An extension length L₂ of the first anode-connected field plate was 2.5μm. The distance L₁ from the anode edge to the cathode edge for a firstgroup of diodes was approximately 10 μm, and for a second group ofdiodes was approximately 15 μm. Reverse bias potentials were applied tothe devices until the devices exhibited breakdown. An extension lengthof the second anode-connected field plate 147 was varied across diodeswithin each group.

Results from the breakdown tests are plotted in FIG. 10. The breakdownvoltage V_(b) observed for the tested devices is plotted as a functionof the extension length L₃ of the second anode-connected field plate147. The results show that a significant improvement in breakdownvoltage is achieved when the extension length L₃ is increased from about0.5 μm to about 1.5 μm. At about 1.5 μm, breakdown voltages of more than1000 V and as much as 1200 V reverse bias were observed for some diodes.Increasing the extension length L₃ further resulted in a reduction ofreverse bias breakdown voltage. For highest breakdown voltages for thetested structure (e.g., in excess of 900 volts reverse bias), thereexists a critical range of field-plate extension lengths L₃ for thesecond anode-connected field plate. In this case, the range is betweenapproximately 1.25 μm and approximately 2.5 μm. The range of lengths maybe increased for lower breakdown voltages. For example, in some cases L₃may be between approximately 1 μm and approximately 3 μm, or in someinstances between approximately 1 μm and approximately 4 μm forbreakdown voltages between about 700 V and about 1200 V reverse bias.

In some implementations, it may be beneficial to have the extensionlength L₃ of the second anode-connected field plate 147 less than anextension length L₂ of the first anode-connected field plate 145.According to some embodiments, the outer edge of the anode, firstanode-connected field plate 145, and second anode-connected field plate147 may lie along a curve 710 (referring to FIG. 7) that bends away fromthe surface of the substrate, e.g., bends away from the barrier layer116.

Some minor differences were observed between the two differentanode-to-cathode distances L₁. A highest breakdown voltage is observedfor the larger anode-to-cathode spacing (15 μm) when the extensionlength L₃ of the second anode-connected field plate is within thecritical range. However, higher breakdown voltages were observed for thesmaller anode-to-cathode spacing at other extension length values.

High-voltage Schottky diodes having three anode-connected field plateshave also been contemplated by the inventors. An example high-voltagediode structure 1100 is depicted in FIG. 11. Numerical simulations werealso carried out for this structure to evaluate the electric field inthe gallium-nitride conduction layer 114. For these simulations, theanode length L_(a) was 10 μm, and the anode-to-cathode spacing L₁ was 10μm. The extension length L₂ of the first anode-connected field plate 145was 2.5 μm, and the extension length L₃ of the second anode-connectedfield plate 147 was 2.5 μm. The thickness t₁ of the first insulatinglayer 120 was 50 nm, and the thickness t₂ of the second insulating layer122 was 450 nm. The thickness t₃ of the third insulating layer 124 wasset at 500 nm for the simulations. In a first set of simulations,electric field values along the gallium-nitride conduction layer werecalculated for a third field-plate extension length L₅ of 2.5 μm, andare plotted in FIG. 12A for four different reverse bias potentials. Thevalues of the peak electric fields E₁, E₂, E₃, and E₄ in thecorresponding regions 310, 320, 330, and 340 of the conduction layer 114can be observed from the plots. For this structure, the outer edge ofthe third anode-connected field plate 149 is close to the cathode, andat high reverse bias the electric field increases significantly near thecathode.

In a second set of simulations, the electric fields in the conductionlayer were calculated for a third extension length L₅ of 1 μm. Byallowing more spacing between an outer edge of the third anode-connectedfield plate 149 and the inner edge of the cathode, the electric fieldnear the cathode reduces. This can be seen from the curves of FIG. 12B.The electric field values shown in FIG. 12B indicate that it is possibleto keep the electric field in the gallium-nitride conduction layer wellbelow its intrinsic breakdown value of 5×10⁶ V/cm at an appliedpotential of −1500 V. This indicates that the device may be capable ofwithstanding reverse bias voltages as much as 2000 V. Reverse biasvoltages up to 1500 V may be applied for long periods of time (e.g.,longer than one second), since the electric field is well below theintrinsic field strength. In some embodiments, an outer edge of any ofthe anode-connected field plates should be spaced horizontally from thecathode by at least 3 microns.

Simulations were also carried out to observe the effect of increasing adistance between the anode and cathode for the diode. In the simulation,the distance L₁ was increased by 50% to a value of 15 μm. However, thisincrease in anode-to-cathode distance had little effect on the peakvalues of the electric fields E₁, E₂, E₃, and E₄. The largest effect ofincreasing the anode-to-cathode spacing appeared as a reduction in theelectric field value near the cathode to a value less than 1×10⁶ V/cm atthe four reverse bias potentials.

According to some embodiments, microfabrication techniques for forminganode-connected field plates may be performed without planarization ofthe substrate after depositions of the anode and first anode connectedfield plate, insulating layers, and subsequent anode-connected fieldplates. Avoiding planarization steps can reduce the time and cost ofdevice fabrication. In some cases, planarization steps (e.g.,chemical-mechanical polishing) may be used after some or all depositionsof the anode-connected field plates and insulating layers.

Example fabrication techniques of the high-voltage Schottky diode willnow be described. Referring to FIG. 13-1A, a wafer may be prepared orobtained that includes a multi-layer stack for a high-voltage Schottkydiode. For example, the wafer may comprise substrate 105, a buffer layer112, a gallium-nitride conduction layer 114, a barrier layer 116, and acap layer 118. The buffer layer 112, gallium-nitride conduction layer114, barrier layer 116, and cap layer 118 may be epitaxially grown onthe substrate or deposited by any suitable process. According to someimplementations, an insulating layer 120 (for example, a silicon nitridelayer) may be deposited over the multi-layer stack using any suitabledeposition process.

A photoresist 1310 may be patterned over the insulating layer 120, asdepicted in FIG. 13-1B. The photoresist may be patterned to open cathodevias 1320 by any suitable photolithography method, such as projectionphotolithography. Subsequently the insulating layer 120, cap layer 118,barrier layer 116, and possibly a portion of the conduction layer 114may be etched anisotropically (e.g., using reactive ion etching) toexpose the conduction layer. In some embodiments, the etch may stop ator part way into the barrier layer. In some implementations, the etchmay continue into the conduction layer 114, as depicted. A conductor1330 (e.g., any suitable cathode composition described above) may bedeposited over the substrate, as depicted in FIG. 13-1C. The firstresist layer 1310 and excess conductor 1330 over the resist may beremoved from the substrate in a lift-off process. The lift-off willleave cathodes 130 on the substrate, as depicted in FIG. 13-1D.

A second layer of resist 1340 may be deposited and patterned to open avia 1342 for patterning the anode. The insulating layer 120 may then beetched anisotropically to form an anode via 1346 and expose the caplayer 118 (or barrier layer 116 if the cap layer is not used). Thesecond layer of resist 1340 may be stripped from the wafer, and a thirdlayer of resist 1350 deposited, as depicted in FIG. 13-1E. A via 1352for forming an anode and anode-connected field plate may be patterned inthe third layer of resist 1350. The via 1352 is larger than the anodevia 1346 in the insulator 120.

A lift-off process may then be carried out to deposit the anode 140 anda first anode-connected field plate 145. An anode deposition may then beperformed, as illustrated in FIG. 13-1F, which deposits any suitableanode conductor 1360 having a composition as described above. Accordingto some embodiments, the deposition also forms a first anode-connectedfield plate 145 that extends beyond the anode 140 over the insulatinglayer 120. The third layer of resist 1350 and excess anode conductor1360 may be removed from the substrate using the lift-off process. Aresulting Schottky diode structure having a single anode-connected fieldplate is depicted in FIG. 13-1G.

Subsequently, a second insulating layer 122 may be formed over thesubstrate. The second insulating layer may be deposited by any suitabledeposition process. The second insulating layer may be coated with afourth photoresist layer 1316, and also patterned to open a via over thefirst anode-connected field plate 145, as depicted in FIG. 13-1I. Thesecond insulating layer 122 may be etched to expose a region of thefirst anode-connected field plate 145. The resist 1316 may be stripped,and a fifth layer of resist 1318 deposited and patterned to open a viafor a second anode-connected field plate, as depicted in FIG. 13-1J. Asuitable conductor for the second anode-connected field plate 147 maythen be deposited as part of another liftoff process, for example, asillustrated in FIG. 13-1K. The fifth layer of resist 1318 and excessconductor 1332 may be removed from the wafer.

The processes of depositing an insulating layer and patterning ananode-connected field plate may be repeated to form one or moreanode-connected field plates over the second anode-connected fieldplate.

Other methods of fabricating a Schottky diode are possible. Analternative method is depicted, in part, in FIG. 13-2A through FIG.13-2F. According to some embodiments, the wafer may comprise a bulksubstrate 105, a buffer layer 112, a gallium-nitride conduction layer114, a barrier layer 116, and an insulting layer 120. A photoresist 1408may be patterned over the insulating layer 120, as depicted in FIG.13-2B. The photoresist may be patterned to open an anode via 1418 by anysuitable photolithography method, such as projection photolithography.Subsequently the insulating layer 120 may be etched anisotropically toexpose a portion of the barrier layer 116 for subsequent deposition ofthe anode. The first resist layer 1408 may be stripped from thesubstrate, and a second layer of resist 1412 deposited and patterned toexpose cathode vias 1422, as depicted in FIG. 13-2C.

The insulating layer 120 at the cathode locations may then be etchedanisotropically, as depicted in FIG. 13-2D to expose at least thebarrier layer 116. In some embodiments, the etch may stop at or part wayinto the barrier layer. In some implementations, the etch may continueinto the conduction layer 114, as depicted. The second resist layer 1412may then be stripped from the substrate.

A liftoff process may then be carried out to deposit the anode andcathodes. For this process, a third resist layer 1414 may be patteredover the substrate to open up cathode vias 1426 and an anode via 1424,as depicted in FIG. 13-2E. The anode via 1424 may be larger than theopened area in the insulating layer 120 from the previous anode viapatterning step. Anode and cathode depositions may then be performed ina single step, as illustrated in FIG. 13-2F. According to someembodiments, the deposition also forms an anode-connected field plate145 that extends beyond the anode 140 over the insulating layer 120. Thethird resist layer 1414 and excess conductor 1430 may be removed fromthe wafer to yield a Schottky diode structure like that depicted in FIG.13-1G. One or more additional anode-connected field plates may be formedover the first anode-connected field plate 145, as described inconnection with FIG. 13-1H through FIG. 13-1K.

According to some embodiments, microfabrication techniques for forminganode-connected field plates may be performed without planarization ofthe substrate after depositions of the anode and first anode-connectedfield plate, insulating layers, and subsequent anode-connected fieldplates. Avoiding planarization steps can reduce the time and cost ofdevice fabrication. In some cases, planarization steps (e.g.,chemical-mechanical polishing) may be used after some or all depositionsof the anode-connected field plates and insulating layers.

A figure of merit for a Schottky diode is an amount of leakage currentthat the device allows to flow under reverse-bias conditions. Ideally, adiode would allow no current to flow when reversed biased. However,diodes typically allow a small amount of leakage current to flow underreverse bias, and this leakage current can contribute to power loss orother deleterious effects in the device or in an instrument in which thediode is used.

Leakage current in a semiconductor diode can be due to several differentcauses. Some of these causes are depicted in FIG. 14. In some cases,surface states 1420 and/or traps 1425 may provide pathways for leakagecurrent. The traps may arise from interfacial defects that form atboundaries between different semiconductor layers. In some cases,leakage current may flow between ohmic contacts 130 a, 130 b via thetwo-dimensional electron gas (2DEG) 214, or via a parasitic channel thatmay form at a boundary between different semiconductor layers. In somedevices, leakage current may flow vertically and laterally between ohmiccontacts (e.g., along a path 1410 depicted in FIG. 14). For example, theleakage current may flow vertically from one ohmic contact 130 b throughone or more gallium-nitride layers and buffer layer to the substrate105, flow laterally along the substrate, and then flow vertically to asecond ohmic contact 130 a. Ohmic contacts 130 a and 130 b may be ohmiccontacts of different devices.

The inventors have studied leakage current in a test GaN Schottky diodedevice, and have found ways to significantly reduce leakage current inthe device. In a first series of measurements that were carried out tobetter understand leakage-current characteristics, the test device wasreverse biased while leakage-current density J_(r) was measured. Thetest device comprised a Schottky diode having a structure like thatshown in FIG. 1A. In the test device, the substrate was bulk silicon(Si), over which nucleation or buffer layers 112 comprising AlN wereformed. An undoped GaN conduction layer 114 having a thickness ofapproximately 0.8 microns was epitaxially grown on the buffer layer, andan undoped barrier layer 116 of Al_(0.27)Ga_(0.73)N was grown over theconduction layer. The thickness of the barrier layer was approximately18 nm. An undoped GaN cap layer 118 having a thickness of approximately2.5 nm was formed over the barrier layer 116. A silicon-nitridepassivation layer 120 was then deposited over the cap layer prior toforming cathode 130 and anode 140 contacts. The cathode edges werespaced approximately one micron from the anode edges. A singleanode-connected field plate was formed over the anode.

Results from reverse-bias, leakage-current measurements are shown inFIG. 15. The applied bias was varied between about 0 volts and about −20volts for six different device temperatures. The reverse-biascharacteristics show that the leakage current in the test device isassociated predominantly with Fowler-Nordheim tunneling andFrenkel-Poole tunneling. Under Fowler-Nordheim tunneling, the leakagecurrent depends predominantly on applied bias and exhibits littledependence on device temperature. The onset of Fowler-Nordheim tunnelingoccurs for an applied bias of about −12 volts for the test device. UnderFrenkel-Poole tunneling, the leakage current exhibits temperaturedependence, and is the predominant form of leakage current for biasvalues between about −2 volts and about −12 volts. The inventors foundthat the leakage current results agree well with Fowler-Nordheim andFrenkel-Poole theories.

Different approaches for reducing leakage current were tried. In somedevices, mesas were formed to help isolate ohmic contacts. For example,trenches were etched around ohmic contacts in non-active regions toreduce unwanted current flow via surface and trapping states and the2DEG between ohmic contacts (e.g., between contacts of adjacent devicesor other device contacts). The inventors found that mesa formation didnot reduce leakage-current flow, and in some cases increasedleakage-current flow. The increase in leakage current was believed to bedue to the generation of more defect states and surface states from theetching.

In some devices, a silicon-nitride passivation layer 1610 may be formedin regions around ohmic contacts 130 a, 130 b, as depicted in FIG. 16A.The passivation layer can passivate the surface states 1420 andappreciably reduce a component of leakage current due to surface-statecurrents. In some devices, ion implantation can be used in non-activeregions of the device alternatively, or in addition to, a passivationlayer 1610. The ion implantation may form electrical isolation regions115 within the semiconductor layers, as depicted in FIG. 16B. The ionimplantation can damage the crystalline structure, and thereby increaseits resistance to leakage current flow.

Several ion species (boron, nitrogen, and phosphorus) were implanted intest devices to determine their effect on leakage current. The inventorsfound that implanted nitrogen provided the largest reduction in leakagecurrent among the different ion species. Additionally, a largerreduction in leakage current can be obtained when the nitrogen isimplanted at a plurality of different energies, so as to extend thedamage well into the conduction layer 114. According to someembodiments, nitrogen may be implanted at two or more different energiesso that the nitrogen implants to depths between about 0.2 microns andabout 0.5 microns below the top surface of the cap layer 118, or belowthe top surface of the barrier layer 116 if a cap layer is not used.

Although surface passivation and ion implantation provided usefulreductions in reverse-bias leakage current, the inventors surprisinglyfound that the largest reduction in leakage current is obtained when apre-treatment process is used prior to deposition of the anode 140. Inconventional anode patterning and referring to FIG. 13-1E, thepassivation layer 120 may be etched to expose the underlying barrierlayer 116 or cap layer 118 for the anode contact. The anode may then bedeposited in electrical contact with the exposed gallium nitride orAlGaN layer, as depicted in FIG. 13-1F. The inventors have found thatprior to depositing the anode, subjecting the exposed layer to an oxygenplasma can significantly reduce reverse-bias leakage current in agallium-nitride Schottky diode. In some embodiments, the exposed cap orbarrier layer is subjected to an O₂ plasma having a pressure betweenabout 0.5 Torr and about 3 Torr, and an applied power between about 0.3kW and about 2 kW. The treatment time may be between about 10 sec andabout 2 minutes, according to some embodiments. In some embodiments, thepressure is about 1.5 Torr with a power of about 1.0 kW for a durationof about 30 sec. Referring to the Schottky diode 1700 in FIG. 17, the O₂plasma treatment is believed to form a thin gallium-oxide layer 1710under the subsequently deposited anode 140. The gallium-oxide layer maybe between about 10 Angstroms and about 50 Angstroms-thick. This thinoxide layer does not appreciably affect forward current flow, butsignificantly reduces reverse-bias leakage current.

In some cases, other gases may be included in the O₂ plasma treatment tohelp passivate the exposed surface. Other gases may include, but are notlimited to nitrogen, hydrogen, argon, and forming gas (a mixture ofhydrogen and nitrogen having about 5% hydrogen).

A measured reduction in reverse-bias leakage current due to O₂ plasmapre-treatment is shown in FIG. 18. Over sixty devices were tested, forwhich conventional techniques were used to open the nitride passivationlayer and form the anode. An exemplary leakage-current curve for thesedevices is plotted as the upper trace 1810 in the graph. Over sixtysimilar devices were also tested, but for these devices an O₂ plasmapre-treatment was used prior to depositing the anode. An exemplaryleakage-current curve for these devices is plotted as the lower trace1820 in the graph. The reduction in leakage current due to the O₂pre-treatment was approximately a factor of 100.

Schottky diodes have been fabricated by the inventors having multipleanode-connected field plates, electrical isolation regions andpassivation layers around ohmic contacts in non-active areas, and usingan O₂ plasma treatment of the cap layer or barrier layer prior todepositing the anodes. Exemplary reverse-bias current curves are shownin FIG. 19 for two typical devices fabricated on different wafers. FourSchottky diodes were tested from each wafer, and produced similarcurves. These diodes are capable of withstanding reverse biases of up toabout 2100 volts. The reverse-bias leakage currents are betweenapproximately 0.1 microamps and approximately 10 microamps over thisrange of reverse bias voltages. The diodes had anode and cathode widthsof approximately 250 microns, so the reverse leakage current valuescorrespond to about 0.4 μA/mm and about 40 μA/mm of anode width W_(a).The difference in value of reverse-bias current for the two curves isbelieved to be due to differences in oxide thickness formed by the O₂plasma treatment for the different wafers. The reverse bias tests werecarried out by applying DC voltages. The diodes were capable ofwithstanding such high reverse-bias voltages for extended periods oftime (greater than one second).

CONCLUSION

The terms “approximately” and “about” may be used to mean within ±20% ofa target value in some embodiments, within ±10% of a target value insome embodiments, within ±5% of a target value in some embodiments, andyet within ±2% of a target value in some embodiments. The terms“approximately” and “about” may include the target value.

The technology described herein may be embodied as a method, of which atleast some acts have been described. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thandescribed, which may include performing some acts simultaneously, eventhough described as sequential acts in illustrative embodiments.Additionally, a method may include more acts than those described, insome embodiments, and fewer acts than those described in otherembodiments.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description is by wayof example only and is not intended as limiting. The invention islimited only as defined in the following claims and the equivalentsthereto.

What is claimed is:
 1. A Schottky diode comprising: a gallium-nitrideconduction layer; a barrier layer formed adjacent to the gallium-nitrideconduction layer; a first cathode and a second cathode spaced apart andformed in electrical contact with the conduction layer; an anode formedadjacent to the barrier layer between the first cathode and the secondcathode; and a gallium-oxide layer formed between the anode and thebarrier layer.
 2. The Schottky diode of claim 1, further comprising oneor more anode-connected field plates electrically connected to the anodeand extending on opposite edges beyond the anode toward the firstcathode and the second cathode.
 3. The Schottky diode of claim 2,wherein the one or more anode-connected field plates comprise: a firstanode-connected field plate electrically connected to the anode andextending on opposite edges beyond the anode toward the first cathodeand the second cathode by a first distance; and a second anode-connectedfield plate electrically connected to the first anode-connected fieldplate and extending on opposite edges beyond the first anode-connectedfield plate toward the first cathode and the second cathode by a seconddistance.
 4. The Schottky diode of claim 3, wherein the second distanceis less than the first distance.
 5. The Schottky diode of claim 2capable of withstanding a reverse-bias voltage of up to 1200 volts. 6.The Schottky diode of claim 2 capable of withstanding a reverse-biasvoltage of up to 2000 volts.
 7. The Schottky diode of claim 6, wherein areverse-bias current is between 0.4 microamp/millimeter and 40microamp/millimeter at a reverse bias of 2000 volts.
 8. The Schottkydiode of claim 6, wherein a thickness of the gallium-oxide layer isbetween approximately 1 nm and approximately 5 nm.
 9. The Schottky diodeof claim 2, wherein the anode, first cathode, second cathode, and afirst anode-connected field plate are formed of a same material.
 10. TheSchottky diode of claim 2, wherein the anode comprises a multilayercomposition selected from the following group: Ni/Pd/Au/Ti, Ni/Pt/Au/Ti,Ni/Ti/Al/W, Ni/W/Al/W, W/Al/W, Ni/Wn/Al/W, WN/Al/W, and Pt/Au/Ti. 11.The Schottky diode of claim 2, wherein an anode-connected field plate ofthe one or more anode-connected field plates comprises a multilayercomposition selected from the following group: Ti/Pt/Au, Al/Cu, orTiN/Cu.
 12. The Schottky diode of claim 1, further comprising a bufferlayer formed on a substrate, wherein a combined thickness of the bufferlayer and gallium-nitride conduction layer is at least 4.5 microns. 13.The Schottky diode of claim 12, wherein the substrate comprises silicon.14. The Schottky diode of claim 12, wherein the barrier layer comprisesaluminum gallium nitride having a thickness between approximately 10 nmand approximately 50 nm.
 15. The Schottky diode of claim 12, furthercomprising a gallium-nitride cap layer formed over the barrier layer.16. The Schottky diode of claim 15, wherein a thickness of thegallium-nitride cap layer is between approximately 1 nm andapproximately 10 nm.
 17. The Schottky diode of claim 12, furthercomprising electrical isolation regions formed adjacent to the first andthe second cathodes, wherein the electrical isolation regions comprisedamaged crystalline semiconductor that includes one or more of thefollowing implanted ion species: nitrogen, phosphorous, boron, andargon.
 18. The Schottky diode of claim 2, further comprising aninsulating layer having a thickness between approximately 100 nm andapproximately 300 nm that is formed between a first extension of a firstanode-connected field plate of the one or more anode-connected fieldplates and the barrier layer.
 19. The Schottky diode of claim 18,wherein the first extension of the first anode-connected field plateextends beyond an outer edge of the anode by at least one micron. 20.The Schottky diode of claim 18, wherein the one or more anode-connectedfield plates includes a second anode-connected field plate having asecond extension that extends beyond an outer edge of the firstanode-connected field plate between approximately 1 micron andapproximately 3 microns.
 21. The Schottky diode of claim 18, wherein anouter edge of the second extension is spaced horizontally from the firstor second cathode by at least 3 microns.
 22. The Schottky diode of claim1, wherein a distance between an edge of the first or the second cathodeand an edge of the anode is between approximately 5 microns andapproximately 25 microns.
 23. The Schottky diode of claim 1, wherein alength of the anode is between approximately 2 microns and approximately20 microns.
 24. The Schottky diode of claim 1, included in a powerconverter.
 25. A method for making a Schottky diode, the methodcomprising: forming a gallium-nitride conduction layer on a substrate;forming a barrier layer adjacent to the gallium-nitride conductionlayer; forming a first cathode and a second cathode spaced apart and inelectrical contact with the conduction layer; forming an anode adjacentto the barrier layer between the first cathode and the second cathode;and forming a gallium-oxide layer between the anode and the barrierlayer.
 26. The method of claim 25, wherein forming the gallium-oxidelayer comprises: opening a via to expose a region of a gallium-nitridelayer at the location of the anode prior to forming the anode; andsubjecting the exposed region to an oxygen plasma for a period of time.27. The method of claim 26, wherein the period of time is betweenapproximately 10 seconds and approximately 120 seconds.
 28. The methodof claim 26, further comprising maintaining a pressure during the oxygenplasma between approximately 0.5 Torr and approximately 3 Torr.
 29. Themethod of claim 26, further comprising forming a gallium-nitride caplayer between the barrier layer and the anode, wherein the gallium-oxidelayer is formed from the gallium-nitride cap layer.
 30. The method ofclaim 26, further comprising forming one or more anode-connected fieldplates in electrical contact with the anode that extend beyond outeredges of the anode.